Protein analysis is the fundamental basis of modern biology research. It centers on antigen-antibody interaction to measure levels of antigen of interest under various medical or experimental conditions. An antigen by definition, is a foreign molecule that, when introduced into the body, triggers the production of an antibody by the immune system. The high specificity of the antibody against a specific antigen makes it a powerful tool in clinical, pharmaceutical and biomedical research. An antigen includes, but not limited to a chemical compound, a peptide, a protein, an RNA, a DNA, a cell (proteins released in situ), or a virus particle (proteins released in situ). The whole molecule of antigen, or part of the molecule, may be introduced into a host animal, such as a donkey, a goat, or a rabbit to generate large quantity of antibody against the introduced antigen of interest. Furthermore, the introduced antigen, or part of the antigen, may have one or several epitopes, thus may generate one or several antibodies against the antigen of interest depending on the number of epitope(s) available.
A typical immunodetection process can be divided into three major steps, including, (i), Sample application, where prepared samples containing an antigen of interest are first bound to a membrane, such as nitrocellulose or PVDF membrane or other solid phase like multi-well plate with protein binding capacity; (ii), blocking/incubation/washing step, which includes multiple sub-steps, where first (a), non-specific protein binding sites on the membrane are blocked using blocking buffer to avoid non-specific protein binding to the membrane; next (b), the membrane is incubated with antibody against antigen of interest to allow for the formation of membrane-bound antigen-antibody complex while unbound antibodies are washed away. In this sub-step, the antibody used may be directly labeled, or indirectly labeled through a secondary antibody, with a reporter enzyme; and (iii), detection, enzymatic reaction is initiated using reporter enzyme coupled with membrane-bound antibody in a reporter assay to give a readout comprising information related to the quantity or quality of the bound immunocomplex on the membrane. In both Dot blot analysis and Western blot analysis, the final result of the immunodetection analysis can be further quantified indirectly through densitometric analysis.
Multiple modifications have been made to this generalized procedure in each individual step. For example, in step (i), there are variations of sample application, including direct application in Dot blot analysis, gel transfer in Western blot analysis and coating of samples in ELISA analysis. Several more modifications have been made in step (ii), including the various procedures and buffer compositions to maximally eliminate direct antibody binding while preserving the formed immunocomplex on the membrane. In most cases, the primary antibody is not directly labeled with reporter enzyme. A reporter enzyme coupled-secondary antibody against the primary antibody maybe needed to label those primary antibodies bound to the antigen of interest on the surface of membrane. In step (iii), besides reporter enzyme, a number of different methods have been used to label the antibody, which, in turn, may lead to various detection methods. For example, the readout may result in color for visual inspection or a chemiluminescence signal that can be detected either through luminometer or X-ray film. The antibody may also be fluorescence-labeled, and the final product is quantified at different wavelength in a microplate reader.
While this generalized description of the immunodetection process is merely illustration of the principles underlying the conventional immunodetection analysis, it is by no means to exhaust all the methods or modifications associated with this process. There are always modifications or procedures not described here, yet consistent with the scope and the spirit of this generalized immunodetection process.
Dot blot analysis is a typical application of the above described immunodetection process, symbolized by the direct application of the prepared samples on to membrane in a dot. However, although this process is simple and fast, its application in biomedical, clinical and pharmaceutical research is limited by its lack of specificity. In multiple cases, antibody used in immunodetection assay reacts with more than one antigen for various reasons. Therefore, the amount of the reporter enzyme associated with bound immunocomplex in a Dot blot analysis cannot reflect reliably the amount of the antigen of interest in prepared samples. Consequently, both Western blot analysis and ELISA assay are more commonly used due to improved specificity.
In Western blot analysis, prepared samples containing the antigen of interest are first separated by their molecular weight though gel electrophoresis, and the separated proteins are transferred through electroblotting step to either nitrocellulose membrane or PVDF membrane. Followed by a typical immunodetection process, the levels of the antigen of interest in the prepared samples are detected on the spot in a typical reporter enzyme-based reaction, and quantified indirectly through densitometric analysis. In this process, the specificity of immunodetection is achieved by both the antigen-antibody interaction as well as the expected molecular weight of the antigen of interest to eliminate any false signals commonly observed in Dot blot analysis. However, in both Dot blot analysis and Western blot analysis, the relative amount of the antigen of interest can only be quantified indirectly through densitometric analysis. Also, in Western blot analysis, the complicated procedures may prevent its application in large-scale analysis in clinical, pharmaceutical and experimental research.
On the other hand, ELISA assay successfully avoids problems associated with both Dot blot analysis and Western blot analysis to allow fast, simple and quantifiable results in a multi-well plate format. The specificity of the assay is achieved by selecting antibody exclusively reacting with the antigen of interest. The high specificity of the antibody-antigen reaction also allows for direct quantification of signal intensity in multi-well plate format. These advantages lead to the wide usage of ELISA techniques in both biomedical and clinical research. Yet, the success of ELISA assay demands high specificity of the antibody, and only those reacting exclusively to the antigen of interest are acceptable for further development. This limitation leads to high developmental cost of successful ELISA assay and limits its availability in the field of biomedical research. In addition, the low binding capacity of the ELISA plate may also significantly limit its usage in the biomedical research field.